The present invention generally relates to a control system of a grinding machine, and more particularly, to an improved control system in which a rotor shaft of a spindle is axially supported on a magnetic bearing.
The present invention further relates to a dressing control system of a grinding stone, and more particularly, to a control system suitable for so-called new wheel dressing on the occasion of replacement of the grinding stone.
In recent years, the accuracy of grind finishing by a grinding machine has increased. With this tendency, a grinding process is effected by rotating the grinding stone at a so-called hypervelocity.
Take an internal grinding machine for instance, the workpiece is ground typically by use of a spindle axially supported on a bearing. More specifically, the workpiece is mounted on a head stock and is then ground to predetermined dimensions by moving the head stock or a grinding stone shaft table or both of them. In this case, a sizing detector serves to detect whether the workpiece is ground to the predetermined dimensions or not. When the amount of variations in the grinding dimensions comes to the predetermined dimensions, the head stock or the grinding stone shaft table is retreated in response to a detection signal of the sizing detector, thereby separating the grinding stone from the workpiece.
There is caused, however, a first drawback that in the case of effecting a grinding process by the prior art system, the grinding stone is moved back when dimensional values reach predetermined dimensions, and scatter appears in the finishing dimensions of the ground workpiece or in a taper thereof due to a time-lag of the table movement associated with the retreat of the grinding stone.
In recent years, the accuracy of grind finishing by a grinding machine has increased. With this tendency, a grinding process is effected by rotating the grinding stone at a so-called hypervelocity. A workpiece is ground by employing a spindle axially supported on a magnetic bearing because of the very high speed rotation.
To be specific, the workpiece is mounted on a head stock and then ground in accordance with predetermined dimensions by moving the head stock or a grinding stone shaft table or both of them. At this time, the head stock or the grinding stone shaft table is moved until the grinding stone touches the workpiece, i.e., the movement during a non-load feed is performed quicker than in the grinding process to reduce a grinding period. The contact between the grinding stone and the workpiece is recognized by detecting ultrasonic frequency oscillation, namely, so-called AE (Acoustic Emission) waves which are generated when the contact takes place, or by detection from an increase in load current of the grinding stone spindle due to a grinding resistance.
There arises, however, a second drawback inherent in the prior art system. When effecting the grinding process by use of the prior art system, and if an end point of the quick feed is detected by the AE waves, the detection undergoes disturbances of mechanical oscillations and thereby becomes inaccurate. Where the end point is detected from the increases in load current, the increment in load current is detected after the grinding stone has come into contact with the workpiece, resulting in a problem of causing a decline in grinding accuracy due to a delay of detection.
Furthermore, an impact of impingement of the grinding stone on the workpiece disadvantageously tends to cause a damage to the grinding stone.
Truing and dressing of a rotatively driven grinding stone provided at a top end of a grinding stone shaft of a spindle has heretofore been performed by a dresser equipped with, for example, a diamond tool.
The dressing process is also effected when replacing the grinding stone. The reason for this will be elucidated as follows. The grinding stone (a new wheel) fitted to the rotor shaft rotates with eccentricity, and a so-called deflection takes place. It is therefore required that the deflection be eliminated to arrange the configuration. A total dressing undercut quantity at that time is controlled to perform a predetermined amount of dressing undercut by moving the dresser or the spindle or both of them so as not to cause a shortage of dress.
In recent years, it has been practiced with a higher accuracy of grinding that the grinding stone is ground by rotating it at a so-called hypervelocity. For this reason, a magnetic bearing type spindle becomes dominant in use, wherein a rotor shaft of a spindle is axially retained by electromagnets.
This, however, creates the following third drawback incidental to the prior art dressing control system. The conventional system is constructed to uniformly effect new wheel dressing control for obtaining a predetermined dressing undercut quantity, causing at least no shortage of dress. As a result, there exists the drawback in which excessive dressing is effected depending on the grinding stone. In this case, abrasions of the dresser become more intense than expected due to excessive dressing, or a dressing time increases.